1. Field of Invention
This invention relates to cyanohydrin derivatives and also to liquid crystal materials containing them. In particular the invention relates to chiral derivatives and ferroelectric smectic liquid crystal compositions containing them.
2. Description of the Prior Art
Ferroelectric smectic liquid crystal materials are known, (e.g. N. A. Clark et al: Appl Phy Lett (1980) 36 899), and exploit the electro-optical properties of the ferroelectric chiral smectic phase. The chiral smectic C (abbreviated S.sub.C * the asterisk denoting chiralty) is most used as it is the most fluid, but other chiral smectic phases e.g. I, F, J, K, G, H or X may also be used.
Although some ferroelectric smectic liquid crystal materials are single compounds it is more usual for such a material to be a composition. Generally such a composition contains one or more compounds which either singly or together shown an S.sub.C phase (termed a smectic "host"), together with one or more chiral (i.e. optically active) compounds which are generally added to induce the smectic material to show a high spontaneous polarisation. Ps. The composition may also contain additives. For example to broaden the Sc* range, to suppress undesirable phases, or optically active compounds to modify the helical pitch of the Sc* phase.
Among the features sought in a good ferroelectric smectic liquid crystal material are low viscosity, fast switching speeds, a broad Sc* phase range, and a high spontaneous polarisation coefficient Ps and a long helical pitch. In view of the high cost of synthesis of optically pure compounds it is also desirable that the chiral compound is cheap.
Optically active compounds used in ferroelectric smectic liquid crystal compositions generally consist of a "core" consisting of a chain of linked cyclic groups, with an optionally active group containing an asymmetric carbon atom at or near a terminal position of the chain. The core is selected from the reasonably well known combinations of cyclic groups which encourage the compound to be miscible with smectic liquid crystal phases. The goal of much research in ferroelectric smectic liquid crystal chemistry is to identify novel optically active groups which may be combined with such cores to form advantageous compounds.
Many suitable optically active compounds are known.
PCT/GB 87/00441 (Agents Ref P0383) and JPA-61-243055 describe compounds of structure ##STR2## where R is alkyl. These compounds show a high Ps. PCT/GB87/00441 has a publication date later than the priority date of this application.